Three South Africans tasted defeat on Sunday as Toulon suffered a 30-6 loss to Clermont at the Stade Marcel-Michelin.
Duane Vermeulen and Juandré Kruger started at No 8 and No 7 respectively for the French club, with Bok wing Bryan Habana coming off the bench.
Clermont led 15-3 at the break, thanks to a penalty try that came two minutes after Toulon flank Liam Gill was sin-binned with a yellow card.
Clermont would score another two tries in the second half, through centres Wesley Fofana and Remi Lamerat, to claim a comfortable win and establish top spot on the Top 14 log with 88 points from 26 matches.
Toulon (82) are second, with Montpellier (81), Racing 92 (81), Toulouse (79) and Castres (71) making up the top six.